The aim of this core is to use a xenotransplantation murine model to provide in vivo readouts for experiments proposed in this program project. Specifically, this model will be used for the following purposes: 1. To optimize proliferation and function of T regulatory cells (Treg) for suppression of alloreactive responses. 2. To perform studies testing the hypothesis that human thymocyte progenitor cells (Tprog) will accelerate human thymopoiesis and peripheral blood T cell responses in immunodeficient mice. 3. To optimize proliferation and function of natural killer (NK) cells for leukemia therapy. These data will in turn inform the design of future clinical trials. The core leader will provide valuable knowledge of the stem cell biology, gene manipulations, murine models, and clinical hematopoietic cell transplantation, all relevant to this application. Core D is unique in its capability to allow two of the three Principal Investigators (who do not have access to preclinical murine models; Project 1 and 3) with a critical shared resource to enhance their scientific program.